Meaning

noun a small glass adequate to hold a single swallow of whiskey

noun any small mast on a sailing vessel; especially the mizzenmast of a yawl

noun larval mite that sucks the blood of vertebrates including human beings causing intense irritation

originFrom jig + -er (agent suffix). Merriam-Webster's Collegiate Dictionary suggests a possible link to Old High German gīga (“fiddle”), for which see modern German Geige.

Etymology from Wiktionary, CC BY-SA 3.0